Drop on demand ink jet technology is widely used in the printing industry. Printers using drop on demand ink jet technology can use either thermal ink jet (TIJ) technology or piezoelectric (PZT) technology. In contrast to thermal ink jet printheads, printheads using piezoelectric technology are more expensive to manufacture but may use a wider variety of inks. Piezoelectric printheads are also relatively larger than thermal printheads for the same nozzle count, which may require a wider spacing of nozzles from which ink is ejected during printing and result in a lower ink drop density and velocity. Low drop velocity decreases the tolerance for drop velocity variation and directionality which, in turn, may decrease image quality and printing speed.
Piezoelectric ink jet printheads may include an array of piezoelectric elements (i.e., transducers). One process to form the array can include detachably bonding a blanket piezoelectric layer to a transfer carrier with an adhesive, and dicing the blanket piezoelectric layer to form a plurality of individual piezoelectric elements. A plurality of dicing saw passes can be used to remove all the piezoelectric material between adjacent piezoelectric elements to provide the correct spacing between each piezoelectric element.
Piezoelectric ink jet printheads can typically further include a flexible diaphragm to which the array of piezoelectric elements is attached. When a voltage is applied to a piezoelectric element, typically through electrical connection with an electrode electrically coupled to a power source, the piezoelectric element bends or deflects, causing the diaphragm to flex which expels a quantity of ink from a chamber through a nozzle. The flexing further draws ink into the chamber from a main ink reservoir through an opening to replace the expelled ink.
Thermal ink jet printheads include a thermal energy generator or heater element, usually a resistor, separated from a nozzle within a nozzle plate by an ink channel. Each heater element may be individually addressed so that an activation of an electrical pulse heats the resistor. The heat is transferred from the heater to the ink, which causes a bubble to form within the ink. For example, a water-based ink reaches a critical temperature of 280° C. for bubble nucleation. The nucleated bubble or water vapor thermally isolates the ink from the heater element to prevent further transfer of heat from the resistor to the ink, and the electrical pulse is deactivated. The nucleating bubble expands until excess heat diffuses away from the ink. During the expansion of the vapor bubble, the ink is forced toward the nozzle and begins to bulge at the exterior of the nozzle plate, but is contained by surface tension of the ink as a meniscus.
When the electrical pulse is deactivated, excess heat diffuses away from the ink and the bubble begins to contract and collapse. The ink within the channel between the bubble and the nozzle begins to move toward the contracting bubble, causing a separation of the ink bulging from the nozzle plate and forms an ink droplet. Acceleration of ink out of the nozzle during the expansion of the bubble provides the momentum and velocity to expel the ink droplet from the nozzle toward a recording medium such as paper in a substantially straight line direction. Once the ink is ejected from the nozzle, the channel may be re-fired after a delay that is sufficient to enable refilling of ink within the channel. A thermal printhead design is discussed in U.S. Pat. No. 6,315,398, incorporated herein by reference in its entirety.
Another type of printhead includes the use of thermo-pneumatic actuators (TPA's). TPA's are similar to thermo-pneumatic (TP) micro-pumps, but do not include inlet and outlet valves. Most printheads rely on surface tension, meniscus pressures, and ink flow impedance to manage fluid flow. In contrast, printheads employing the use of TPA's use a membrane to separate an active or pumped fluid (e.g., an active fluid such as an ink which is pumped out of the printhead) from a working or trapped fluid that is sealed within each actuator. Because the ink itself may have less than optimal thermal characteristics, the working fluid is selected for its improved thermal performance during operation of the device. The membrane isolates the working fluid and prevents it from mixing with the pumped fluid. A lower half of the TPA (the portion beneath the membrane) includes a resistive heater and the working fluid, while the upper half of the TPA (the portion between the membrane and the nozzle plate) includes the pumped fluid. The heater, which, in an array comprising a plurality of heaters, can be individually addressed and activated so that it is energized to heat the working fluid to a point close to its critical temperature. As a result, nucleation sites appear in the working fluid that coalesce to form rapidly growing vapor bubble as described for the bubbles in a thermal ink jet but formed in the working fluid. The bubble grows, deflects the membrane and the active fluid is pressurized in its fluid path. Accordingly, the membrane is an actuatable membrane. The pressure pulse causes the active fluid to move or transmit pressure in a useful way such as being ejected from a nozzle and onto a recording medium such as paper. A similar configuration used for a hybrid ink jet print head is described in U.S. Pat. No. 5,539,437, which is incorporated herein by reference in its entirety.
Thermo-pneumatic actuators are used as fluidic pumps as well as droplet ejectors but are limited in their actuation frequency because of thermal buildup. For example, operation of such devices is accompanied by a baseline temperature rise until the heat input is matched by the heat loss to the environment. At this point, the device reaches an elevated steady state temperature. However, if the boiling point of the working fluid is below the steady state temperature then actuation will cease, rendering the actuator inoperable. That is, as the actuator is cycled, excess heat raises the temperature of the working fluid until its boiling point is exceeded at which point it completely vaporizes rather than only a portion to form the bubbles that act against the membrane. Accordingly, thermo-pneumatic actuation is limited in cycling frequency due to the length of time it takes for the device to cool off between cycles.
A printhead device design and manufacturing process that allows for operation at elevated temperatures to improve frequency response would be desirable.